This invention relates generally to converter topologies useful for direct current (DC) transmission, and more particularly to a medium voltage direct current (MVDC) transmission system for sub-sea loads.
Transportation of electrical power to oil and gas sub-sea electrical equipment often requires high power to be transported over long distances. Transmission to sub-sea equipment is used to supply the power from an onshore utility to a point where the power is distributed among individual loads. Generally, a step down transformer is implemented in order to bring the high voltage level of the transmission stage to a lower voltage level for a distribution stage to individual units of the electrical equipment. Distribution distances are typically shorter than the transmission distance; and the voltage levels to be supplied to individual loads or load clusters are lower than the voltage levels of the transmission stage. Typically, power on the order of 50 megawatts is transmitted by high voltage alternating current (AC) transmission cables to a high voltage transformer, thereafter stepping down the voltage for a medium voltage AC distribution system.
Typically, in AC power systems, medium voltage is used in the power distribution system and includes nominal voltages of 12, 24, and 36 kV. The next commonly used nominal voltage is 72 kV (which is not considered to be a medium voltage but rather a high voltage for power transmission). Since the phase-to-phase voltages in a 36 kV three-phase-system reach amplitudes of more than 50 kV, DC transmission systems with +/−50 kV are considered herein to fall within the scope of MVDC power transmission systems for sub-sea loads. Transmission voltages of +/100 kV or higher are therefore considered herein to be outside the scope of MVDC power transmission systems for sub-sea loads. Such voltages are used in HVDC transmission projects where high power is transmitted over long distance (e.g., in transmitting 100 MW or 200 MW over a distance of 100 or 200 km).
AC transmission provides technical challenges for applications where bulk power is transmitted over long cables. Capacitance causes charging current to flow along the length of the AC cable. Because the cable must carry this current as well as the useful load current, this physical limitation reduces the load carrying capability of the cable. Because capacitance is distributed along the entire length of the cable, longer lengths produce higher capacitances, thus resulting in higher charging currents. As the cable system design voltage is increased to minimize the line losses and voltage drop, the charging current also increases.
Typically, multiphase booster pumps require electrically driven motors delivering a shaft power between 2 MW and 6 MW. Future offshore oil and gas resource installations will require pump installations at distances above 50 km from the shore. Such distances require a high voltage power transmission; however, high voltage AC transmission is very costly when supplying single sub-sea pumps or clusters of a few sub-sea pumps only, where the power to be transmitted is at or below 20 MW.
Further, sub-sea motors driving a gas compressor typically have a higher nominal power (e. g., in the order of 10 or 15 MW). As such, sub-sea compression clusters may be required to transmit a total power in the order of 50 to 100 MW over a distance of 100 or 200 km. The transmission of high power over a distance of more than 100 km and distributing the power sub-sea is very challenging with AC transmission and distribution systems because of the high charging currents and the high number of components involved in the distribution system.
In general, DC transmission can be achieved more efficiently over long distances than AC transmission. High voltage (HV) DC transmission typically requires the usage of power electronic converters in the transmission systems that are capable of converting between HVAC and HVDC. In conventional HVDC converter topologies, each switch of the converter is designed to handle high voltages. The converter nominal voltage may range from tens-of-kilovolts to hundreds-of-kilovolts, depending upon the application. Such switches are typically configured utilizing a plurality of series connected semiconductor devices (e.g., such as insulated gate bipolar transistors (IGBTs) and thyristors). Because of the size and the high number of components involved, conventional HVDC terminals are not well suited for sub-sea installations.
Converters are also required on the load side of a power distribution system when supplying variable speed motors in addition to the power conversion required for HVDC transmission. Typically, a high voltage transformer is used to step down the voltage from the AC or DC transmission level to the voltage level used in the AC power distribution system. On the load side of the distribution system, the converters convert the power from fixed frequency AC voltage (stepped down from the transmission system) to a variable frequency AC voltage of individual motors that must be controllable over a wide speed range when driving sub-sea pumps or compressors.
A need therefore exists for a power transmission system for sub-sea loads that substantially reduces the costs and increases the reliability beyond that achievable when using known HVAC and HVDC transmission techniques, particularly for applications using single sub-sea motors at a distance above 50 km.